Conventional elevator installations have safety circuits which consist of safety elements connected in series. These safety elements monitor, for example, the state of shaft doors or car doors. Such a safety element can be a door contact, a lock contact, a buffer contact, a flap contact, a sensor, an actuator, a travel switch, an emergency stop switch, etc. An open contact shows that, for example, a door is open and a potentially impermissible door state has arisen. If with the opened contact an impermissible open state of the door is now identified then the safety circuit is interrupted, which has the consequence that a drive or brake, which influences the travel of an elevator car, brings the elevator car to a stop.
A safety system or a safety circuit for an elevator installation is known from the document EP 1638880, which comprises a control unit as well as at least one safety element and a bus as communications network. The bus or safety bus enables communication between the at least one safety element and the control unit. The safety element can, for example, monitor the state of shaft and car doors. Moreover, the at least one safety element consists of a receiver and a transmitter.
The document EP 1427662 describes a safety system with safety bus. The safety bus is used in order to enable a secure and reliable monitoring of shaft doors of an elevator installation.
The document EP 1427660 describes a safety system with safety bus which permits evaluation of the state of car and shaft doors.
The understanding of bus or bus system is, for example, as described in the book ‘Bussysteme, Parallele and serielle Bussysteme, lokale Netze’, by Georg Färber, R. Oldenbourg Verlag Munich Vienna, 1987, ISBN 3-486-20120-4.